VFD Voltage/RPM fluctuating

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stutaylo
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VFD Voltage/RPM fluctuating

Post by stutaylo »

Hi all,

I am bench testing my Acorn board and have been succesful for the most part.
I have completed bench/comms test - no issues.
I have completed spindle output bench test - voltage steady and within +/- 5%

I am using a Fuling (chinese) 5.5kw VFD and an Air cooled 4.5Kw spindle - Max speed 18,000rpm, Max frequency 300hz.

Report is attached.

Power supply for Acorn and VFD are on separate circuits, both with filters on the AC inlets

I currently have it wired so Acorn Output 8 is set to 'VFD Enable', this is wired to the 'Enable' pin on the VFD, which puts it in RUN mode when active.
I am then using the +/-10V control from the Acorn to control the spindle speed. This works perfectly, until I drop below about 12500rpm spindle speed/220hz on vfd/7.2V on spindle output from Acorn - The spindle speed will fluctuate up and down about 500rpm.

I'm sending a simple 'M3 Sxxxxx' in the MDI to control speed

When I put a voltmeter on the +/-10V spindle speed supply, the voltage is fluctuating with the speed change in the spindle, indicating that the VFD is just doing what It's told.

With spindle power supply disconnected, the voltage remains steady on the spindle control voltage at all speeds - indicating it may be interference from the VFD


I then tried putting a jumper across the 'Enable' pins on the VFD, and disconnected the 'VFD enable' wiring. The issue disappeared and spindle rpm & control voltage was very steady at all speeds - this narrows it down to something happening when the Output 8 relay is used to close the 'VFD Enable' pins. The speed control is exactly as one would expect

I have also tried Output 1 on the Acorn with no change



Not sure quite where to go from here, I need something to send the VFD a spindle enable to use the VFD properly, but can't/won't run it with such a speed fluctuation


I have also attached a photo of my wiring layout - I am very aware that there are no E-stops wired in, and that it's not tidy as yet.


Thank you,

Stuart
Attachments
IMG_20210205_145832.jpg
report_E415F6F7B8A9-1124204137_2021-02-05_14-28-51.zip
(618.91 KiB) Downloaded 74 times
tblough
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100327
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7804732B977B-0624192192
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Re: VFD Voltage/RPM fluctuating

Post by tblough »

Make sure the analog out lines are shielded and properly grounded using a separate twisted pair cable (two wires, nothing else). You can then try a 0.1 microFarad capacitor (100pF) across the analog output if the problem is still there.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
stutaylo
Posts: 11
Joined: Fri Jan 01, 2021 4:08 pm
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CNC12: Yes
CNC11: No
CPU10 or CPU7: No
CNC Control System Serial Number: none

Re: VFD Voltage/RPM fluctuating

Post by stutaylo »

Ok thats good info, thank you

Can you clarify your wording around the grounding and twisted pair cable?

I have some shielded cable, which has two separately shielded cables (image attached)

Do I ground both ends of the shielding, or just one end, using twisted pair cable?

Appreciate the help,

Stu
Attachments
ED4988DE-95C3-43F4-9604-72CF29D91429.jpeg
tblough
Posts: 1706
Joined: Tue Mar 22, 2016 10:03 am
Acorn CNC Controller: Yes
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MPU11 & GPIO4D -w/ 3rd Party Drives: Yes
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CNC12: Yes
CNC11: No
CPU10 or CPU7: No
CNC Control System Serial Number: 100505
100327
102696
103432
7804732B977B-0624192192
Location: Boston, MA
Contact:

Re: VFD Voltage/RPM fluctuating

Post by tblough »

Something like this: https://www.digikey.com/en/products/det ... 00/7009257
8760MN-008100.png
I normally try to ground the shield on the end where the signal originates; in this case the Acorn end. Oftentimes this is overridden by which end has a suitable ground point nearby. I don't like to go more than a few inches to find a shield ground so will adjust which end I ground accordingly. You also want to maintain the twist right up to the terminations. Don't strip them back 6" and untwist them, and don't run them through terminal blocks in between.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
ChrisAttebery
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Re: VFD Voltage/RPM fluctuating

Post by ChrisAttebery »

I had the same problem when I hooked up my VFD. I used a single piece of 4 conductor shielded stepper motor cable for the analog input and the enable. The VFD didn’t like that. My cure was to run a separate cable for the digital and analog inputs. I made sure that I grounded the shields at both ends. It works perfectly now.

FWIW: I used a single cable on my old setup and it worked fine. I’d love to see the output of the relays on an oscilloscope.
Muzzer
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Re: VFD Voltage/RPM fluctuating

Post by Muzzer »

tblough wrote: Fri Feb 05, 2021 7:20 am You can then try a 0.1 microFarad capacitor (100pF) across the analog output if the problem is still there.
Not a biggie but 0.1uF is actually 100nF or 100,000pF. A 100pF cap wouldn't have any effect that you'd notice.
tblough
Posts: 1706
Joined: Tue Mar 22, 2016 10:03 am
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CNC Control System Serial Number: 100505
100327
102696
103432
7804732B977B-0624192192
Location: Boston, MA
Contact:

Re: VFD Voltage/RPM fluctuating

Post by tblough »

I'm old. Senior moment.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
stutaylo
Posts: 11
Joined: Fri Jan 01, 2021 4:08 pm
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DC3IOB: No
CNC12: Yes
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CPU10 or CPU7: No
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Re: VFD Voltage/RPM fluctuating

Post by stutaylo »

Thanks a lot guys, I will replace the unshielded 4 conductor cable with two separate shielded, earthed cables

The only controller I've built prior to his was a Gecko G540 and Mesa 5i25, fairly different setups but apparently quite robust against EMI

Loving the Acorn so far though, simple to setup and seems like a decent 'light industrial' control
stutaylo
Posts: 11
Joined: Fri Jan 01, 2021 4:08 pm
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CNC11: No
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CNC Control System Serial Number: none

Re: VFD Voltage/RPM fluctuating

Post by stutaylo »

Ok, so first move was to replace cable with shielded, grounded cable. This improved the issue but did not fix it completely. Best results were when shielding was grounded at both ends.

I then put a 0.1uF capacitor across the analog outputs on the Acorn end, which fixed the problem completely. Steady voltage and RPM at all speeds

Thank you all for your help
ChrisAttebery
Posts: 78
Joined: Mon Dec 07, 2020 2:20 pm
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Re: VFD Voltage/RPM fluctuating

Post by ChrisAttebery »

That’s great news. I added a 1mf tantalum cap to my analog output too.
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